As a current type light emitting device, an Organic Light emitting Diode (OLED) is increasingly applied to a high performance Active Matrix Organic Light Emitting Device Display (AMOLED). With an increasing in the display size of a conventional Passive Matrix OLED (PMOLED), the conventional PMOLED requires a shorter single pixel driving time for displaying, and thus a transient current is needed to be increased, which renders an increasing in power consumption. Meanwhile, an application of the large current causes a voltage drop of the Indium Tin Oxide (ITO) line to decrease too much, rendering the operation voltage of OLED too high and in turn an efficiency of OLED is lower. An Active Matrix OLED (AMOLED) inputs an OLED current via switching transistors by progressive scanning for display, which can solve the above problems very well.
In designing of a pixel circuit of the AMOLED, a problem mainly to be solved is non-uniformity in luminance of OLED devices driven by pixel driving units in AMOLED.
Firstly, in AMOLED, a pixel driving unit is constituted by thin film transistors (TFTs) to supply a corresponding driving circuit to a light emitting device. As the inventor noticed, low temperature poly-Si Thin Film Transistors (LTPS TFTs) or Oxide TFTs are mostly adopted. As compared to the conventional amorphous-si TFT, LTPS TFT and Oxide TFT have a higher mobility and more stable characteristics, and thus are more suitable to be used in an AMOLED display. However, due to limitations of a crystallization process, LTPS TFTs, which are manufactured on a glass substrate with a large area, have non-uniformity in electrical parameters such as a threshold voltage, the mobility, etc, and such non-uniformity may result in variances in driving currents and luminances among OLEDs which can be perceived by human eyes, i.e., Mura phenomenon. Although a process for the Oxide TFTs shows a better uniformity, similar to a-Si TFTs, the threshold voltage of the Oxide TFT may drift under a high temperature or under a case that the Oxide TFT is supplied a voltage for a long time. Due to different display pictures, drifts of threshold voltages of TFTs in respective areas on a panel may be different from each other, which may cause variances in display luminance. Such variances in display luminance often render an image sticking phenomenon since such display luminance difference has a relation to a previously displayed image.
Since the OLED light emitting device is a device driven by a current (current-driven device), the threshold characteristic of the driving transistor in a pixel driving unit for driving the light emitting device to emit light has a great effect on the driving current and the ultimate display luminance. The threshold voltage of the driving transistor will drift under a voltage stress or light illumination, which causes the non-uniformity in the luminance of the resulted display.
In addition, in order to remove the effect caused by the variances in threshold voltages of the driving transistors, the existing pixel circuits are commonly designed to have a complex structure, which may render a low good manufacturing production rate of pixel circuits of AMOLED.
Therefore, in order to solve the above problem, there is a need for providing a pixel driving unit, a driving method and a pixel circuit in embodiments of the present disclosure.